T8: ISO 4217 per-currency decimals + format_money()
- Add CURRENCY_DECIMALS dict (JPY=0, BHD/KWD/OMR=3, rest=2) and DEFAULT_CURRENCY_DECIMALS=2
- Add CurrencyConverter.format_money() staticmethod with ROUND_HALF_UP; symbol-prefix for known, code-prefix for unknown
- Replace 14 :,.2f literals across services.py / info.py / config_cmd.py with format_money() calls
- Refactor 2 i18n templates (transaction_add.ok, recurring_add.ok): {amount:.2f} -> {amount_str}; callers pass pre-formatted string
- New _account_currency() helper for transaction currency lookup
- Extend test_currency.py: 8 new tests (JPY/BHD/USD/CHF/negative/zero/float-int/lowercase)
- 75/75 tests pass; 0 :,.2f literals remain in infrastructure/cli/
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